The objective of the proposed studies is to develop and characterize animal model for studying the progression of estrogen-dependent cancer to estrogen independent state. The need for developing such animal model is obvious because practically all invasive human breast cancers, although they may initially respond to hormonal therapy, will progress to hormone independent state, and there is no suitable and well-characterized animal model available to study this process in details. A few years ago, it was shown that although over 90% of N-methyl-N-nitrosourea (MNU)-induced rat mammary cancers either become impalpable or show more than 50% reduction in size after ovariectomy, a high percentage of these tumors show re-growth with prolonged time after ovariectomy. This animal model will be used in this proposal for further development and characterization. It is hypothesized that mammary cancers that show a renewed growth after ovariectomy initially continue to depend on estrogen for growth by developing an increased sensitivity to estrogen and by enhancing local synthesis of estrogen after estrogen deprivation, but gradually lose their need for estrogen to acquire complete estrogen independence. The following two specific aims are proposed: 1. To investigate what changes are seen in gene expression when mammary cancers progress from a state of ovarian-dependent growth to re-growth in the absence of ovarian hormones. Here, microarray analysis will be used to determine global changes in gene expression of mammary tumors when they progress from hormone-dependent to hormone-independent state. 2. To investigate the importance of estrogen/estrogen receptor activity in mammary cancers of ovariectomized animals. In this specific aim, it will be investigated if the estrogen receptor continues to be involved in growth regulation of mammary cancers that grow after ovariectomy of the tumor-bearing animals.